1. Field of the Invention
The present invention relates to an infrared detector for hydrogen fluoride gas and more specifically to a dual-beam gas detector which compares light transmissiveness through a gas that may contain hydrogen fluoride gas and in which the transmission of a light beam though a reference filter having a wavelength which is not highly absorbed by hydrogen fluoride gas is compared with the transmission of a light beam through an hydrogen fluoride sensing filter having a wavelength which is highly absorbed by hydrogen fluoride gas.
2. Description of the related art including information disclosed under 37 CFR .sctn..sctn.1.97-1.99.
Hydrogen fluoride gas leaks from hydrogen fluoride alkylation units at refineries are of great concern to the petroleum industry. Also, hydrogen fluoride gas at aluminum smelting sites is of concern to the aluminum industry.
This is particularly so because very small amounts of hydrogen fluoride (HF) gas can cause irritation of the nose, eyes and skin, as well as respiratory irritation. Slightly larger amounts, e.g., 50 ppm-m for a period of 30-60 minutes can cause severe irritation of the eyes, burning of the skin, and lung and cardiovascular collapse leading to death.
Accordingly, at sites where HF gas may be present, periodic tests are conducted to determine the presence of HF gas.
One technique for testing the air is to suck air past a sensitized tape and then to test the tape with colorimetry. This technique is very slow.
Another technique is to utilize an electrochemical gas monitoring device such as a gas monitoring instrument of the type sold by International Sensor Technology c,f Irvine, CA which utilizes "self-consuming" electrochemical and catalytic sensors, and includes a heater and a collector embedded in a metal oxide material Such instruments are expensive, have a slow response time and have a drop-off in accuracy and sensitivity at low concentrations of HF gas, e.g., down to 50 ppm-m.
Also, heretofore, Wright & Wright, Inc dual-beam hydrocarbon detectors have been utilized by the petroleum industry for sensing the presence of hydrocarbons for the purpose of stopping product loss and preventing contamination of the adjacent environment. Such detectors are much less expensive than the electrochemical gas monitoring instruments but, heretofore, have not been utilized for detecting HF gas.
Further, it is known from U.S. Pat. No. 4,701,395 to provide an IR-sensitive photoconductive member comprising a charge transport layer, a semi-conductor superlattice of alternating layers of amorphus silicon and amorphus germanium or other alloys, each 5-100 angstroms thick and having at least 10 periods; a first blocking layer adjacent the first layer of the superlattice and a support layer adjacent the first layer on the blocking layer. Such a photoconductive member exhibits high photosensitivity at wavelengths up to 0.9 microns and is used for the infrared detection of oil spills or hydrocarbons.
Additionally, French Patent No. 2 304 862 discloses a torch-burning hydrocarbon system using a selective infrared radiation pyrometer and a thermistor. A range of radiation from the flame in a torch-burning waste gas is selected together with the value of luminance of this radiation
The luminance value selected is at a level below that corresponding to the appearance of black smoke.
Radiation is detected continuously and an electrical signal is generated which is a function of the luminance of the radiation. This signal is used for the automatic control of the injection of steam into the burning waste gas for the purpose of reducing the accidental emission of black smoke.
As will be described in greater detail hereinafter, the hydrogen fluoride gas detector of the present invention differs from the previously proposed techniques for sensing HF gas by utilizing a Wright & Wright, Inc. dual-beam detector modified to include selected reference and sensor filters and enhanced calibration circuitry, thereby to provide an inexpensive, fast response, and high sensitivity HF gas detector which can provide an alarm signal for concentrations of HF gas from 30-50 parts per million meters.